Meshblock Conflation

Overview

The NGD network needs to be linkable to the new CRN network. This includes individual arcs and BB (meshblock) units.

The CRN and NGD network differences are not limited to the quantity of arcs, but also the shape and alignment of those arcs. This is why the networks cannot be simply linked arc-to-arc, but require more detailed conflation to reduce alignment differences. A threshold of 80% is currently used to determine acceptable network alignment and classify individual meshblock polygons (BBs) as conflated or unconflated.

Definition

conflated: An NGD BB which has >= 80% of its area contained within a single CRN BB.

Threshold Strictness

It will not always be possible to achieve the conflation threshold without introducing needless complexity to the CRN. Therefore, the threshold should be treated as a soft restriction and lower values are acceptable so long as the NGD BB can be guaranteed to link to a single CRN BB based on majority area occupation, as defined above for conflation.

The required actions to resolve unconflated BBs should follow those outlined in the various validations in Meshblock Creation.

Resources

CLI Tool:

src/conflation/conflate_meshblock.py

Output (see data/crn.gpkg):

  • Basic metrics output to console.

  • Updated source layer: crn_<source>

  • New (CRN) BB layer: <source>_meshblock

  • Current (NGD) BB layer: <source>_meshblock_ngd

Editing Environment:

data/editing_meshblock_conflation.qgz

Editing Process

Editing process overview.

Figure: Editing process overview.

Cardinalities

Definition

Cardinalities: In a database context, cardinalities refer to the numerical relationships between records of two or more datasets.

Cardinalities overview.

Figure: Cardinalities overview. Direction: CRN (blue) - to - NGD (pink).

Cardinalities Explained:

one-to-one:
Explanation:

Networks align.

Action:

None.

many-to-one:
Explanation:

CRN is more detailed.

Action:

None.

one-to-many:
Explanation:

NGD is more detailed.

Action:

Create new BOs and / or copy missing NGD roads.

many-to-many:
Explanation:

Networks conflict.

Action:

Create new BOs.

QGIS Project

Explanation of Layers

QGIS table of contents.

Figure: QGIS table of contents.

Layers:

crn:

Primary layer representing NRN roads, NGD BOs, and added NGD roads.

ngd_road:

NGD roads for reference and identifying roads missing from the CRN which are required for conflation.

meshblock_ngd:

Current NGD BB layer, classified by conflation validity status and labelled with a conflation percentage.

meshblock:

New CRN BB layer generated from crn, classified by conflation validity status and labelled with a conflation percentage.

Esri Satellite:

Reference WMS for recent imagery context.

Example Output

qgis_example_output

Conflation Scenarios

Converting NGD roads to BOs

If an NGD road needs to be converted to a BO, copy and paste the NGD feature(s) into the CRN data and set bo_new=1. The script will then automatically set the correct segment_type for these features.

Adding new BOs (completely new arcs)

If a network difference prevents an CRN BB from being properly conflated, you may need to add a new arc to the CRN data. After creating the arc, either set bo_new=1. The script will then automatically set to remaining attributes.

Scenario: Missing NGD Roads

scenario_missing_ngd_roads

Scenario: Missing False NGD Road (1)

scenario_missing_false_ngd_road_1

Scenario: Missing False NGD Road (2)

scenario_missing_false_ngd_road_2

Scenario: Misaligned Networks

Note

Misaligned networks are difficult to resolve since there is no clear nor obvious solution. If the CRN and NGD BBs are not too different in shape, try modifying the BO shape slightly or rerunning the script with a slightly lower threshold value. Larger differences may require adding NGD roads as BOs and / or adding completely new BOs, thereby segmenting the data until the conflation threshold is satisfied.

scenario_misaligned_networks_1
scenario_misaligned_networks_2
scenario_misaligned_networks_3

Scenario: Misaligned Networks - Additional Examples

scenario_misaligned_networks_additional_examples

Scenario: Structural Differences

Note

A structural difference is where the CRN and NGD have different network representations (not the same as offsets / alignment issues).

scenario_structural_differences_1
scenario_structural_differences_2

Scenario: Isolated Blocks

scenario_isolated_blocks

Scenario: Correction of NGD Road

Note

Inaccurate / low resolution roads can be corrected based on the imagery layer, if desired, so long as the conflation threshold remains satisfied. However, this is not always possible (as shown below) and is not the objective of this task.

scenario_correction_of_ngd_road

Progress

Note

This section is temporary and will be removed once this task is completed.

Meshblock conflation workload.

Figure: Meshblock conflation workload (error counts).

Meshblock conflation progress map.

Figure: Meshblock conflation progress map as of June 8, 2023.

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Meshblock Creation

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Arc Linkage